The Sinclair SPJ-1 is a 2-axis, 2 button analogue joystick originally designed for the Sinclair PC200. The PC200 (PC-8) compatibility is also stated in the official Original Arnold V Specs. The 2 buttons are likely to be digitial.
It can be used on the Amstrad PLUS Analog port.
The SPJ-1 (Sinclair Professional Joystick) was produced to accompany Amstrad's unsuccessful Sinclair PC200 (an old PC with 8086 processor). The SPJ-1 was a fully-fledged analogue joystick, with potentiometers used to measure input (rather than switches as in its digital precessors). It was probably a rebadged PC-standard device. The SPJ-1 was, like its host machine, a flop, and today appears to be exceptionally rare.